a. Field of the Invention
The present invention relates to a method of and apparatus for detecting one or more foreign substances on a semiconductor LSI wafer or a mask pattern, and especially to a method and apparatus suitable for the high-speed and high-sensitivity detection of one or more fine foreign substances on a patterned wafer or the like in an intermediate step of an LSI fabrication process.
b. Description of Related Art:
Inspection of foreign substances on a patterned wafer in an intermediate step of an LSI fabrication process is indispensable for the improvement of yield and reliability. Conventional methods and apparatus for the automatic detection of foreign substances on a patterned wafer were realized using a detection technique which makes use of polarized light as disclosed in Japanese Patent Application Laid-Open Nos. 149829/1980, 101390/1979, 94145/1980 and 30630/1981. The principle of this detection technique will next be described with reference to FIGS. 21A, 21B and 21C and FIGS. 22A and 22B.
FIGS. 21A, 21B and 21C are schematic illustrations of the principle of detection of a foreign substance by a first example of the conventional methods for the detection of foreign substances and an apparatus therefor. An S polarized laser beam 15c is irradiated horizontally onto a wafer 7 as shown in FIG. 21A. Light 12p is then reflected without any change in polarization from a pattern 2 on the wafer 7, said pattern 2 extending at a right angle to the illumination beam 15c and advances, still in the S polarized state, to an object lens 6. An analyser 151 is arranged with its axis of polarization extending perpendicularly to the polarization of the reflected light 12p, so that the reflected light 12p is subjected to extinction and is not allowed to reach a detector 20. In the case of light 12p reflected from the pattern 2 which extends at an angle with respect to the illumination beam 15c as shown in FIG. 21B, the light 12p does not enter the object lens 6 and is not detected. When the illumination beam 15c which is travelling in a Y direction is irradiated onto a foreign substance 13 on the wafer 7 as depicted in FIG. 21C, the polarization of the illumination beam is changed and reflected light 12 is produced as P polarized light (a sort of elimination phenomenon of polarization). Since the P polarized light can pass through the analyser 151, the foreign substance 13 can be detected by the detector 20.
FIGS. 22A and 22B are a perspective view and discrimination ratio graph of an optical system in the first example of the conventional methods and apparatus for the detection of foreign substances. When the oblique angle .phi. of the S polarized laser beam 15c from a laser beam source 15 is successively changed relative to the wafer 7, there is obtained a graph of measurement data of discrimination ratio which is the ratio of the intensity of the light 12 scattered by a 0.5 .mu.m foreign substance of 1 .mu.m foreign substance to the intensity of the light 12p scattered by the pattern, said foreign-substance-scattered light 12 and said pattern-scattered light 12p being both detected by the detector 20 via the object lens 6 and the analyser 151, as shown in FIG. 22B. Using the variations in the output characteristics of the foreign substance 13 and the pattern 2 according to the oblique angle .phi., the oblique angle .phi. is changed in a suitable range, followed by detection and comparison.
According to a second example of the conventional methods and apparatus for the detection of foreign substances on a patterned wafer, as illustrated in U.S. Pat. No. 4,740,079 issued Apr. 26, 1988, to the present assignee and incorporated herein by reference, not only polarized laser illumination having a large scattering loss but also illumination having a small scattering loss, namely, two types of illumination are applied to a foreign substance. Making use of the fact that the former illumination tends to produce scattered light at the foreign substance while the latter illumination tends to give off scattered light at the pattern, fine foreign substances can each be detected more stably with higher sensitivity by detecting the ratio of a signal corresponding to the former scattered light to a signal associated with the latter scattered light. The second example uses plural photoelectric solid pickup element arrays of the type where the size of each of the pixels of their light-receiving portions is on the order of 5.times.5 .mu.m.sup.2 as converted to the corresponding value on the surface of a sample. Signals outputted from the individual elements are subjected to concurrent comparison processing, whereby high-sensitivity detection of foreign substances can be achieved without deterioration of high-speed performance. The principle of this detection method for foreign substances will next be described with reference to FIGS. 23 through 26D.
FIG. 23 is a perspective view of an optical system according to the second example of the conventional methods and apparatus for the detection of foreign substances. The conventional method depicted in FIG. 23 uses variations in the output characteristics of the foreign substance 13 and pattern 2 according to the oblique angle .phi. shown in FIGS. 22A and 22B. For example, a low-angle S-polarized illumination beam 15c (wavelength: .lambda..sub.1) from a laser light source 15L and a condenser lens 15bL and a high-angle S-polarized illumination beam 11 (wavelength: .lambda..sub.2) from a laser light source 15H and a condenser lens 15bH are irradiated onto the same point on the sample. Of light reflected from the point, a P polarization component of scattered light 12 obtained by way of the object lens 6, a color-separation prism 150 and detection elements 151L,151H is solely detected by photoelectric solid pickup element arrays 20L,20H. Their output signals V.sub.L,V.sub.H are compared at analog division comparators 100 and then converted to binary signals at binarizing circuits 101. The binary signals are then outputted through an OR gate/OR circuit 22.
FIGS. 24A through 24E are schematic illustrations of output signals and the like in FIG.23. FIG. 24A is a side view showing the scattered lights 12p,12 occurring upon irradiation of the laser beam 15c at a lower oblique angle onto the sample (Si wafer) 7 having thereon the pattern (POLY-Si) 2 and the foreign substances 13a,13b of different sizes. FIG. 24B(a) shows the waveform of the output signal V.sub.L at that time, while FIG. 24B(b) depicts the waveform of a binary signal Sd obtained in accordance with a threshold value V.sub.O. FIG. 24C is a a side view in which the laser beam 11 is irradiated at a higher oblique angle onto the sample (Si wafer) 7 having thereon the pattern 2 and the foreign substances 13a,13b. FIG. 24D illustrates the waveform of the output signal V.sub.H at that time. Further, FIG. 24E(a) shows the waveform of the output signal ratio V.sub.L /V.sub.H and FIG. 24E(b) depicts the waveform of the binary signal Sd obtained in accordance with a threshold value m.
FIGS. 25A through 25H and FIGS. 26A through 26D are optical path diagrams of the polarization in FIG. 23. fig. 25A shows the above illumination and detection conditions in a simplified schematic manner. The output signals V.sub.L and V.sub.H are obtained as a result of detection of the P polarized components of scattered lights S(L) and S(H), which have been obtained upon irradiation of the sample 7 by the S-polarized illumination beams 15c,11, by the color-separation prism 150 and the analysers 151L,151H. The illumination and detection conditions are not limited to the conditions depicted in FIG.25A but various illumination and detection conditions as shown in FIGS. 26A through 26D are also usable. In these drawings, the illumination and detection conditions L for highlighting the foreign substances 13a,13b can be either the detection of the P-polarized component by the S-polarized illumination S(L) or the detection of the P-polarized component by the P-polarized illumination P(L). Reasons for this are set out in U.S. Pat. No. 4,740,079 referred to above.
On the other hand, the illumination and detection conditions H for highlighting the pattern 2 can be any conditions other than both the above illumination and detection conditions L, so that it is not absolutely necessary to use polarized light by laser beam illumination. Namely, incoherent light such as that available from the use of a conventional halogen lamp or the like can be employed. This light is indicated by S+P in FIGS. 26A through 26D.
As the above color-separation prism 150, it is possible to use the dichroic prism (or mirror) disclosed in Japanese Patent Application Laid-Open No. 149829/1980 or 43559/1981 or a combination of a light-splitting prism (semitransparent mirror) and a color filter or interference filter. When the laser beam sources 15L,15H are two lasers of different wavelengths selected from He-Ne laser (.lambda.: 6,328 .ANG.), GaAlAs laser diodes (.lambda.: 7,800-8,300 .ANG.), InGaAsP laser diode (.lambda.: 13,000 .ANG.) and Ar laser (.lambda.: 4,580 .ANG., for example), their laser beams can be condensed on the surface of the sample 7. As a result, high illuminance can be obtained so that the detection of the scattered lights 12p, 12 can be stabilized further. As has been described above, it is the essential requirement for the second example of the conventional art that the illumination and analysis conditions L for highlighting foreign substances are either the detection of P polarized light component by the S-polarized illumination S(L) or the analysis of P polarized light component by the P-polarized illumination P(L) and the foreign-substance-highlighting illuminations L have a wavelength .lambda..sub.1 or .lambda..sub.2 different from that of the pattern-highlighting illumination H.
FIG. 27 is a detailed circuit diagram of a signal processing circuit which contains the analog division comparators of FIG. 23. In FIG. 27, output signals V.sub.L,V.sub.H from the detectors 20L,20H are processed pixel by pixel i-n at the analog division comparators 100 so that output signal ratios V.sub.L /V.sub.H are calculated. At the binarizing circuits 101, these output signal ratios are converted to binary signals in accordance with the threshold value m. The OR gate 22 carries out the logic OR between the corresponding binary signals from the binarizing circuits 101. Whenever the logic OR results in "1", it is stored in a foreign substance memory 23. The analog comparison method of FIG. 23 (FIG. 27) will next be described in further detail with reference to FIG. 28A through FIG. 30B.
FIGS. 28A, 28B and 28C schematically illustrate the experimental results under the illumination and analysis conditions shown in FIG. 23. FIGS. 28A and 28B are plan and side views of the reflected lights (scattered lights) 12p,12 from the circuit pattern 2 and foreign substance 13 on the sample 7 when illuminated by the beams 15c,11, respectively. FIG. 28C is a diagrammatic representation of the experimental data, in other words, the relation between the output signals V.sub.L and V.sub.H under the above conditions. Incidentally, the object lens 6 is provided with a lens frame 6a in FIG. 28B. With respect to the scattered light 12p from the pattern 2, the output signals V.sub.L,V.sub.H of the scattered light 12p from the pattern 2 are measured while successively turning the pattern 2 over angles .eta. from an angular point perpendicular to the projecting direction of the illumination beams 15c,11 onto the surface of the sample wafer 7. Using standard particles of 0.5, 0.7, 1 and 2 .mu.m (no turning is needed in this case), the output signals V.sub.L,V.sub.H of the scattered lights 12 from the foreign substance 13 are measured. FIG. 28C diagrammatically illustrates the experimental data, i.e., the relation between the output signals V.sub.L and V.sub.H. It is understand from this diagram that the, at any desired angles .eta. of the pattern 2, the output signal ratios V.sub.L /V.sub.H (indicated by circles) from the pattern 2 are smaller than the threshold value m of a discrimination threshold line V.sub.L /V.sub.H =m (the inverse of the inclination of the broken line in the diagram) but the output signal ratios V.sub.L /V.sub.H (indicated by dots) from the standard particles of 0.7-2 .mu.m and an actual larger foreign substance U as the foreign substance 13 are greater than the threshold value m of the discrimination threshold line. The sample wafer 7 was turned successively over the angles .eta., because the sample wafer 7 has on the surface thereof the pattern 2 extending at various angles .eta. and the foreign substance 13 must be stably detected in distinction from the pattern 2.
With reference to FIGS. 29A through 30B, a description will next be made of a method for discriminating the pattern 2 and the foreign substance 13 from each other by means of an electric circuit which has been constructed by taking into consideration the characteristics of the output signals V.sub.L, V.sub.H from the pattern 2 and the foreign substance 13 depicted in FIG. 28C.
FIG. 29A is a characteristic diagram of the output signal ratios V.sub.L /V.sub.H illustrated in FIG. 28C. FIG.29B is a circuit diagram showing an exemplary discriminator which makes use of the analog division comparator 100 and is adapted to realize the characteristics of the output signal ratios V.sub.L /V.sub.H shown in FIG. 29A. The output signals V.sub.L,V.sub.H are processed at the analog division comparator 100, so that the output signal ratio V.sub.L /V.sub.H is calculated. At the binarizing circuit 101, the ratio is then converted to a binary signal on the basis of the threshold value m and, when the output signal ratio V.sub.L /V.sub.H is greater than m, "1"* is outputted. When the output signal V.sub.H is low, the calculation error of the output signal ratio V.sub.L /V.sub.H becomes larger so that the calculation results become unstable (for example, V.sub.L /V.sub.H =.alpha. when V.sub.H is zero). As a method for overcoming this problem, it is only necessary to set the calculation results of the output signal ratio V.sub.L /V.sub.H effectively at "1" whenever V.sub.L &gt;V.sub.TH, V.sub.TH being the value of an output signal V.sub.L corresponding to the foreign substance 13 as large as 0.5 .mu.m, in FIG. 29A. This can be realized by a binarizing circuit 104 for the output signal V.sub.L, said binarizing circuit 104 having a threshold value V.sub.TH, and an AND gate 103 having inputs "1"* and "1"**.
FIG. 30A is a characteristic diagram of differences V.sub.L -V.sub.H of the output signals shown in FIG. 28C. FIG. 30B is a circuit diagram of an illustrative discriminator which uses an analog subtraction comparator 105 for obtaining the characteristics of the output signal differences V.sub.L -V.sub.H of FIG. 30A. In this case, the discrimination threshold line is set at m=1 (inclination: 45.degree.) by adjusting the intensity of either one of the illumination beams 15c, 11 by the illuminations L,H and/or the gain of an unillustrated output amplifier of either one of the pickup element arrays 20L,20H. Whenever the output from the binarizing circuit 104 is "1"** (V.sub.L &gt;V.sub.m), the output "1"* of the subtraction result V.sub.L -V.sub.H from the analog subtraction comparator 105 for the output signals V.sub.L,V.sub.H of FIG. 30B is outputted as an effective "1" from the AND gate 103. Incidentally, instead of the analog division or subtraction comparison shown in FIG. 29B through 30B, the output signals V.sub.L,V.sub.H can be subjected to digital calculation after they have been subjected to A/D conversion.
It is the overlooking of foreign substances that is involved as a first problem in the second example of the prior art. To detect the 0.5 .mu.m foreign substance in distinction from the pattern on the basis of the measurement results of FIG. 22B, it is desirable to set the oblique angle .phi. of the illumination L at about 0.degree.-5.degree. degrees and the oblique angle .phi. of the illumination H at 10.degree. or greater and to compare signals of scattered lights. To detect 0.5 .mu.m foreign substances at a high S/N ratio, an object lens 6 having a diameter large enough to permit effective condensing of scattered light is required. The lens frame 6a of FIG. 28B therefore becomes large. As a result, oblique angles .phi. of 10.degree. lead to interference of the illumination beam 11 with the lens frame (metal frame) 6a so that no sufficient discrimination performance can be obtained. As a result, the 0.5 .mu.m foreign substance shown in FIG. 28C is overlooked. The foregoing are the experimental data obtained by using, as models of foreign substances, spherical particles called standard particles. The actual foreign substance Q of the submicron order shown in FIG. 28C is also overlooked.
It is a lack of sufficient intensity by light scattered from a foreign substance that is involved as a second problem in the second example of the prior art. As will be described subsequently with reference to FIG. 13(a), where the illumination beam 15c from the oblique illumination L for highlighting the foreign substance 13 is an S-polarized beam, it is only the P-polarized component 12(P) out of the light 12 scattered from the foreign substance that is detected by the detector 20L in the second conventional example. Because the P-polarized component 12(P) is considerably smaller than the S-polarized component 12(S) retaining its polarized state, it is impossible to obtain any sufficient quantity of light at the detector 20L. It is therefore impossible to obtain any sufficient S/N ratio for the output signal V.sub.L (P) shown in FIG. 13(c). Low-pass filtering is hence applied to the output signal V.sub.L to reduce the noise N, so that a long time is required for the detection of the foreign substance.
It is a lack of sensitivity for fine foreign substances on a mirror-finished surface that is involved as a third problem in the second example of the prior art. For the detection of foreign substances on an LSI wafer or the like, it is generally required to be able to detect not only foreign substances of 0.5 .mu.m and greater on the pattern but also foreign substances of 0.1 .mu.m and greater on the mirror-finished surface and the mirror-finished film. As will be described subsequently with reference to FIGS. 14A and 14B, in the case of a foreign substance 13 of the submicron order, forward scattered light 12f is strong but side scattered light 12e impinging upon the object lens 6 is weak, in the light 12 scattered from the foreign substance upon exposure to the low-angle illumination beam 15c(11). It is therefore desirable to enlarge the angular aperture of the object lens 6 with a view toward detecting a part of the forward scattered light 12f. For the reasons described above in connection with the first problem, however, a limitation is imposed on the size of the angular aperture .alpha.. No sufficient quantity of light can therefore be obtained from 0.1 .mu.m foreign substances on a mirror-finished sample, whereby the output signal V.sub.L (V.sub.H) shown in FIG. 13(b) cannot be detected.
If the points of detection by the two detectors 20L,20H on a sample are not registered in the second example of the prior art as depicted in FIG. 41, the detectors may not use corresponding pixels to detect the pattern 2 and the foreign substances 13a,13b or the timing of detection by a pixel of one of the detectors may not coincide with the timing of detection by the corresponding pixel of the other detector. This may lead to a deterioration in the detection sensitivity for foreign substances. To avoid these problems, precise positional or sensitivity matching is needed between the two detectors 20L and 20H. Obviation of such positional or sensitivity matching, if feasible, is believed to lead to simplification of detection procedures for foreign substances.
Further, the detectors 20L,20H are employed in the second example of the prior art so that an amplifier and the like are required for each of the detectors. If detection is feasible by using only one detector, the circuit size can be reduced correspondingly. This is certainly preferable.